I can't find the Mkiii specs as yet but I be very surprised if they
were not vastly more suited to a sub. will keep you posted,
but I believe the Shiva suits around a 50L sealed box and a
100L to 150L reflexed / vented box.

KEFs are used in the Perfectionist Audio subwoofer and a number of other respectable designs, that's why I didn't dismiss this unit out of hand. All of them are TL designs without the PR though (but then there's always an option to ditch the PR...)